/*

 * Copyright (c) 2014, Oracle America, Inc.

 * All rights reserved.

 *

 * Redistribution and use in source and binary forms, with or without

 * modification, are permitted provided that the following conditions are met:

 *

 *  * Redistributions of source code must retain the above copyright notice,

 *    this list of conditions and the following disclaimer.

 *

 *  * Redistributions in binary form must reproduce the above copyright

 *    notice, this list of conditions and the following disclaimer in the

 *    documentation and/or other materials provided with the distribution.

 *

 *  * Neither the name of Oracle nor the names of its contributors may be used

 *    to endorse or promote products derived from this software without

 *    specific prior written permission.

 *

 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"

 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE

 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE

 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE

 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR

 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF

 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS

 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN

 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)

 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF

 * THE POSSIBILITY OF SUCH DAMAGE.

 */

package org.openjdk.jmh.samples;


import org.openjdk.jmh.annotations.*;

import org.openjdk.jmh.runner.Runner;

import org.openjdk.jmh.runner.RunnerException;

import org.openjdk.jmh.runner.options.Options;

import org.openjdk.jmh.runner.options.OptionsBuilder;


import java.util.concurrent.TimeUnit;


@State(Scope.Thread)

@BenchmarkMode(Mode.AverageTime)

@OutputTimeUnit(TimeUnit.NANOSECONDS)

public class JMHSample_10_ConstantFold {


 /*

  * The flip side of dead-code elimination is constant-folding.

  *

  * If JVM realizes the result of the computation is the same no matter what,

  * it can cleverly optimize it. In our case, that means we can move the

  * computation outside of the internal JMH loop.

  *

  * This can be prevented by always reading the inputs from non-final

  * instance fields of @State objects, computing the result based on those

  * values, and follow the rules to prevent DCE.

  */


 // IDEs will say "Oh, you can convert this field to local variable". Don't. Trust. Them.

 // (While this is normally fine advice, it does not work in the context of measuring correctly.)

 private double x = Math.PI;


 // IDEs will probably also say "Look, it could be final". Don't. Trust. Them. Either.

 // (While this is normally fine advice, it does not work in the context of measuring correctly.)

 private final double wrongX = Math.PI;


 @Benchmark

 public double baseline() {

  // simply return the value, this is a baseline

  return Math.PI;

 }


 @Benchmark

 public double measureWrong_1() {

  // This is wrong: the source is predictable, and computation is foldable.

  return Math.log(Math.PI);

 }


 @Benchmark

 public double measureWrong_2() {

  // This is wrong: the source is predictable, and computation is foldable.

  return Math.log(wrongX);

 }


 @Benchmark

 public double measureRight() {

  // This is correct: the source is not predictable.

  return Math.log(x);

 }


 /*

  * ============================== HOW TO RUN THIS TEST: ====================================

  *

  * You can see the unrealistically fast calculation in with measureWrong_*(),

  * while realistic measurement with measureRight().

  *

  * You can run this test:

  *

  * a) Via the command line:

  *    $ mvn clean install

  *    $ java -jar target/benchmarks.jar JMHSample_10 -i 5 -f 1

  *    (we requested single fork; there are also other options, see -h)

  *

  * b) Via the Java API:

  *    (see the JMH homepage for possible caveats when running from IDE:

  *      http://openjdk.java.net/projects/code-tools/jmh/)

  */


 public static void main(String[] args) throws RunnerException {

  Options opt = new OptionsBuilder()

          .include(JMHSample_10_ConstantFold.class.getSimpleName())

          .forks(1)

          .build();


  new Runner(opt).run();

 }

// 本例介绍了 constant-folding，即常量折叠，上述代码的 measureWrong_1 和 measureWrong_2 中的运算都是可以预测的值，所以也会在编译期直接替换为计算结果，从而导致基准测试失败，注意 final 修饰的变量也会被折叠。
}